On the significance of the respiratory exchange ratio after different diets during exercise in man

Abstract

Total respiratory exchange ratio (R) was compared to the respiratory exchange ratio over the legs (RQ_L) during exercise after different diets, to evaluate if R (which represents a mean for the whole body) can be used to estimate the relative proportions of fat and carbohydrate oxidation in exercising muscle. One important prerequisite for this is a steady state acid base balance. 7 subjects were studied at rest and during the later part of a 25 min exercise (65% of Vo₂ max) on two occasions, the first preceded by a fat rich diet and the second by a carbohydrate rich diet. Oxygen uptake, R and arterial‐femoral venous differences for [O₂]. [CO₂], PCO₂ and pH and arterial concentrations for lactate and β‐hydroxybutyrate were measured. Respiratory exchange ratio over the exercising legs (RQ_L) and ventilationloxygen uptake were calculated. Arterial pH, PCO₂, lactate and β‐hydroxybutyrate as well as specific ventilation attained steady levels during the later part of exercise after both diets. Although arterial lactate and β‐hydroxybutyrate differed between the diets. the arterial pH and specific ventilation were the same. Both R and RQ_L were higher after the carbohydrate than after the fat diet and there was no systematic difference between R and RQ_L. Therefore, it seems likely that R estimates the proportion of fat and carbohydrate oxidation in skeletal muscle during submaximal exercise after extreme diets.